1. Field of the Invention
The present invention relates to an optical sensor used for an image reader of an image processing device such as a facsimile apparatus or a digital copying machine and, more particularly both to a method of driving a thin film transistor (hereinafter called "TFT") type optical sensor which is so constructed that a gate electrode is provided through an insulating layer for a thin film semiconductor layer, to the drive unit.
2. Related Background Art
As a construction example of the TFT type optical sensor, FIG. 1 shows a typical plan view, and FIG. 2 shows a sectional view along line 2-2' in FIG. 1. In FIGS. 1 and 2, numeral 1 denotes a substrate of glass, etc., numeral 2 denotes a gate electrode, numeral 3 denotes a gate insulating layer, numeral 4 denotes a photo conductive semiconductor thin film, and numeral 5 denotes an ohmic contact layer used to make an ohmic contact between the semiconductor thin film 4 and both a source electrode 6 and a drain electrode 7.
When such a TFT type optical sensor is normally used in a charge accumulation mode as an image reader, a bias voltage VB is applied to the gate electrode as shown in FIG. 3 for stable operation. In FIG. 3, a sensor battery VS is connected to a drain electrode, and an accumulation capacitor C is connected to a source electrode. A charge stored in the accumulation capacitor C is discharged to a load resistance RL by a transfer switch SW.
Even in this TFT type optical sensor, the electrical quality may change when the interface or the surface condition of the thin film semiconductor layer is affected by the film forming process or an etching process in the production process. Also when it is produced on a large area of substrate, etc., the distribution in these processes may cause great distribution to the electrical quality.
To give such an example, when a TFT type optical sensor has been formed at intervals of about 20 mm on a large area (320.times.256 mm) of glass substrate a shown in FIG. 4, a light current Ip and dark current Id (when a voltage between gate voltage and source voltage Vgs=0 V) of a TFT type optical sensor on the A-A' line in FIG. 4 are distributed as shown in FIG. 5, for example, and the S/N may also decrease depending upon the location.
Even in a driving method in which a bias voltage is applied to the gate electrode for stable operation, there may be problems at issue in the S/N decreases and a distribution occurs in the sensor output value.
For this reason, a method to adjust the gate bias voltage is used in accordance with such an array position of a TFT type optical sensor as shown in the Japanese Patent Appln. Laid-Open No. 63-92153 previously filed by the present applicants. The equivalent circuit of this is shown in FIG. 6.
In FIG. 6, Sl,l to Sm,n are TFT type optical sensors, CSl,l to CSm,n are accumulation capacitors, Ul,l to Um,n are transfer TFTS, and Vl,l to Vm,n are reset TFTS.
The above-mentioned element group is divided into m blocks n by pieces, and is matrix connected to (m+1) pieces of gate lines and n pieces of signal lines.
A driver unit 11 applies a voltage to gate lines VGl to VGm+1 in order, a signal processing unit 12 takes out a signal voltage from signal lines Sl to Sn, and a control unit 13 applies a control voltage to sensor gate lines VBl to VBm. Also VS is a sensor bias voltage, VR is a reset voltage for the accumulation capacitor, and CLl to CLm are load capacitors.
Although the above-mentioned problems at issue can be almost solved by this system, new problems at issue such as adding out-going wiring for the gate bias and requiring an adjustable external power supply occur, preventing the cost of the image reader from being reduced. Further since the gate bias voltage value should be adjusted each time, it increases the production man-hour, hindering any the cost reduction.